1. Field of the Invention
The present invention relates to a photovoltaic element such as a solar cell and to a method of manufacturing the same. More specifically, the present invention relates to a photovoltaic element having a heterojunction constituted by combining a crystal semiconductor and an amorphous semiconductor, and to a manufacturing method thereof.
2. Description of the Background Art
Recently, solar cells employing crystal silicon such as single crystal silicon and polycrystalline silicon have been much studied and practically implemented as photovoltaic elements. Among these, a solar cell having a heterojunction constituted by combining amorphous silicon and crystal silicon has been attracting attention, as the junction can be formed through a low temperature process of at most 200.degree. C. and it provides high conversion efficiency.
Further, it has been known that by inserting a thin intrinsic amorphous silicon at the heterojunction interface between the crystal silicon and the amorphous silicon, properties of the solar cell can be significantly improved.
Meanwhile, in manufacturing a conventional photovoltaic element in which amorphous semiconductor and crystal silicon are combined, different from a photovoltaic element consisting only of crystal silicon, it is necessary to form an amorphous silicon layer or a conductive thin film for forming a junction on the crystal silicon substrate.
Conventionally, when such a film is formed on the substrate, manufacturing method such as plasma CVD, sputtering or vapor deposition has been utilized. Accordingly, the amorphous silicon layer or a conductive thin film extends not only over the surface of the substrate but also to the side surface or further to the rear surface, possibly causing short-circuit of the element.
Accordingly, as disclosed, for example, in TAIYO ENERGY KOGAKU (Solar Energy Engineering), issued by BAIFUKAN, pp. 221 or in TAIYO ENERGY HENO CHOSEN (Challenge to Solar Energy), issued by SEIBUNSHA, p. 50, film formation has been performed while covering peripheral portions of the substrate by means of a mask formed of metal, for example, in order to prevent the above described extension of the film to the side surface or to the rear surface.
FIG. 8 is a cross section showing a structure of an example of a photovoltaic element manufactured through the conventional method using such as a mask.
Referring to FIG. 8, the photovoltaic element includes, on a surface of an n type crystal silicon substrate 7, an intrinsic amorphous silicon layer 8, a p type amorphous silicon layer 9, a conductive thin film 3 and a collecting electrode 4 formed in this order, and on a rear surface of crystal silicon substrate 7, an intrinsic amorphous silicon layer 8, an n type amorphous silicon layer 10, a conductive thin film 3 and a back electrode 5 formed in this order.
However, in the conventional photovoltaic element shown in FIG. 8, amorphous silicon layers 9 and 10, conductive thin film 3 and the like are not formed at the peripheral portion A of substrate 7. Therefore, this portion A is a non-generating portion or a high resistance portion, which means that it is a loss portion. As a result, effective area is reduced and conversion efficiency is low.
Further, as conductive thin film 3 also serves as an anti reflection film, when there is a portion on which conductive thin film 3 is not formed, there is a reflection loss, and light confining property is decreased.
Generally, for mass production, the number of steps employing mask should be as small as possible. In other words, the conventional manufacturing method using a mask leads to poor productivity.